Glaucoma is the world's the second leading cause of irreversible blindness. The World Health Organization (WHO) estimated the incidence of blindness due to glaucoma to be 4.4 million people worldwide in 2002. Intraocular pressure (IOP) is the sole proven modifiable risk factor for the development and progression of glaucomatous optic neuropathy. Medical therapy is aimed at lowering IOP in order to prevent or slow progression.